If your experience confined you to the virtual plains of the blogosphere, you could be forgiven for thinking that Andrew Keen was one of the most unpopular people on the planet. One blogger - on Keen's own website - recently described him as "a professional mental prostitute of the establishment". New media guru and Guardian columnist Jeff Jarvis has called him "a mastodon growling against the warm wind of change". Keen recently introduced himself on the Today programme as "the antichrist of Silicon Valley". So what has he done?

He's written a book, The Cult of the Amateur, with the no-messing-about subtitle "How Today's Internet is Killing Our Culture and Assaulting Our Economy". It may sound like a technophobe's bible, but Keen himself is no Luddite. He has his own blog and his own podcast programme, AfterTV.com. He was one of the pioneering entrepreneurs of the first internet boom, with his own start-up, Audiocafe.com, and one of the first to go bust when the bubble burst (to hear him tell it, he actually went bust before the bubble burst). To internet enthusiasts Keen isn't just a heretic; he's an apostate.

The Cult of the Amateur is a broadside attack on Web 2.0, a term we may hastily define here as that growing sector of the internet which serves mainly as a platform for user-generated content, including sites such as MySpace, Facebook, Typepad, Blogger and YouTube. The main thrust of his argument is that all this home-made content - blogs, podcasts, amateur videos and music - is an inadequate replacement for mainstream media. It may be a harmless, even occasionally enriching addition, but we can't have both, because the former is swiftly killing off the latter. Thanks to Web 2.0, newspapers, record companies, movie studios and traditional publishers are on the verge of extinction, he says. Along the way he also finds time to bash Second Life, online gambling, copyright theft and porn.

His attack even encompasses one of the web's more widely admired experiments - Wikipedia, the online encyclopedia written and edited by anyone who wants to have a go, on the principle that the crowd possesses an aggregate wisdom all of its own. "To my mind Wikipedia is not wise," says Keen. "It's dumb. Not necessarily because all its contributors are dumb, but because if you don't have an editor in charge, and you don't have singular voices, then the intellectual quality of what the crowd produces is very low."

Until recently the Wikipedia entry for Andrew Keen informed readers that, in addition to coming from Golders Green, London, having an academic background and being an outspoken critic of Web 2.0, he was also "a child actor who found fame in a series of soup commercials". This isn't true; the sentence was inserted deliberately by the host of a Radio 3 show prior to an appearance by Keen, to show how easily the accuracy of Wikipedia can be undermined. This bit of factual vandalism remained for 12 days before it was removed - 11 days longer than an emendation from June 5, which replaced the entire first paragraph with the words "Andrew Keen IS a dumb motherfucker".

So it goes with Keen and the people he sometimes calls "the denizens of the cyberswamp"; he baits them and they rise. He belittles the contributions of Amazon reviewers, and they give his book one resentful star out of five. He compares bloggers to a million monkeys at a million typewriters, and they respond with reams of invective. He criticises Wikipedia for its vulnerability, for its excessive faith in the wisdom of the crowd, and some anonymous user - as if to prove the point - defaces his entry.

Keen has a particular knack for phrasing his criticisms in a way that allows every blogger to feel personally slighted. Part of this stems from his use of the word "amateur", which seems to dismiss the contribution of anyone who isn't getting paid for their trouble.

"I think that's probably a fair criticism," he says. "I'm sure there is some quite good writing on the internet, written by people who don't care about making money out of it, and who have something interesting to say."

At the same time he remains "very uncomfortable with the radical altruism - in some ways it's a legacy of the hippy culture - that lies at the heart of Web 2.0; the idea that we're all happy to give it away. I don't think that's the case. I think the majority of us need to work for money."

Keen claims he isn't really going after the bloggers so much as the influential idealists who actually run Web 2.0. "My real targets are what I would call the libertarians on the right and the left," he says. To Keen, the "democratised" web is actually a form of oligarchy, the product of an unholy alliance between old counterculturalists ("fat guys with beards, basically") and free-market fundamentalists (he offers Chris Anderson, author of The Long Tail, as an example). The former group, he says, reject "all forms of external authority"; the latter believe "that if you just leave everything alone it will work itself out".

One inviolable tenet of this twin-track libertarian ethos, according to Keen, is a misplaced faith in the integrity of the amateur - the citizen journalist, the self-published author, the mash-up musician - and a generic distrust of expertise. One does indeed find this attitude mirrored all over the net, where people frequently post sayings such as "Amateurs built the ark; professionals built the Titanic". Mainstream media is seen as corrupt, compromised, lazy and fearful, while Web 2.0's army of amateur content-generators is dynamic, honest, worthy and wise. In Keen's estimation this idea isn't just absurd - it's dangerous. "For these Generation Y utopians," he writes, "every posting is just another person's version of the truth; every fiction is just another person's version of the facts."

Keen's argument strikes a chord with certain professions, particularly librarians, editors and educators. Keen's critics, on the other hand, see him as defending a largely abandoned redoubt: old media, with its outmoded "gatekeepers" and structural hierarchies. Others see him as a man embittered by the failure of his start-up company, who resents the subsequent success of the Web 2.0 pioneers. When he gave a talk at the ICA last month, someone stood up and accused him of writing "an extended whine about why people like you are no longer in charge of this culture". This remark drew applause, but Keen says the hostile reception was nothing like as bad as he gets in America.

Is he surprised by the strength of feeling?

"No, I expect it."

Does it ever bother him?

"Everybody, I guess, wants to be loved," he says, laughing. But Keen is so ready to make provocative statements, even when they might undermine his overall argument, that his blogger-baiting begins to look like a marketing strategy. "I don't know if it necessarily sells books," he says, "because I don't think bloggers read." Another statement, you might think, to launch a thousand outraged paragraphs.

In fact, the book, he insists, isn't really about the internet. It's more about personal responsibility: "It's not against technology. It's simply saying that we make technology and we need to control it. When we look at the internet we're looking at ourselves."

Keen concedes that he made some mistakes in setting out his case which probably haven't helped win over the opposition. "I think I idealised mainstream media ... I concentrated on the good things. I didn't write about the Sun newspaper. I didn't write about Fox." His opponents have been able to pick holes in his arguments - indeed there is a website devoted to doing so - but he says the book is a polemic primarily designed to start the conversation, and in that respect it has been a success. "Even my biggest enemies agree that there is a need to have this discussion."

He also accepts that the clock cannot be turned back, that user-generated content will continue to dominate the web, not because it's noble or truthful or authentic, but because it's free. "No one pays for content any more," he says. And if no one is willing to pay for content, then it simply becomes a publicity tool, another form of promotional giveaway. "That's what's going to happen with books," says Keen, "and even with movies. In a funny kind of way you could argue that that's what my book is. It's a way to build my brand so that people will pay me to make speeches."

And with that he goes off to tell an audience of internet advertisers that they've got it all wrong. His talk is entitled "The message is dead: how Web 2.0 is reducing all marketing to spam".

· The Cult Of The Amateur by Andrew Keen is published by Nicholas Brealey, price £12.99. To order a copy for £11.99 with free UK p&p go to theguardian.com/bookshop or call 0870 836 0875